The present invention relates generally to etchant formulations for the etching of various types of silicon oxide, and particularly to etchant formulations that have the capability to etch different types of silicon oxide at different relative etch rates than prior art etchants; formulations of the present invention include a chelating agent, a fluoride salt, a glycol solvent and, optionally, an amine.
Several different types of silicon oxide are utilized in the fabrication of semiconductor devices. Two of the most common types are referred to as xe2x80x9cthermal oxidexe2x80x9d and xe2x80x9cBPSG.xe2x80x9d Thermal oxide is typically composed of pure silicon dioxide and is utilized when an insulating layer is required. For example, thin xe2x80x9cgatexe2x80x9d layers of thermal silicon oxide are often utilized to separate conducting layers from each other. Thermal oxide layers on semiconductor wafers are usually prepared by high temperature oxidation of surface silicon on a polysilicon wafer. BPSG layers are comprised of silicon oxide which has been doped with boron and phosphorus. These layers serve the purpose of xe2x80x9cgetteringxe2x80x9d alkali metal ion contaminants which could otherwise migrate into underlying layers and adversely affect electrical properties of the layer materials, causing device reliability degradation. BPSG layers are typically created by deposition of a suitable precursor material via atmospheric chemical vapor deposition (xe2x80x9cCVDxe2x80x9d) followed by thermal treatment densify and planarize the layer. PSG layers serve the same gettering functions as BPSG layers but lack boron.
Various prior art formulations, as discussed herebelow, for etching various types of silicon oxide compounds typically etch BPSG layers and PSG layers more rapidly than thermal oxide layers, which can sometimes be a limiting factor in the fabrication of semiconductor devices.
Aqueous solutions of hydrofluoric acid ranging from very dilute solutions ( less than 1% by weight) to concentrated solutions (up to 49% by weight). The disadvantage of these solutions, in addition to lack of selectivity, is extremely high etch rates and metal corrosivity, making them suitable only for bulk etching of silicon oxide in the absence of exposed metal.
Nonaqueous solutions of hydrogen fluoride, Gajda, U.S. Pat. No. 4,230,523, discloses an etchant solution of hydrogen fluoride in an organic solvent such as glycerin or another polyhydric alcohol. The solution is utilized to etch silicon dioxide without etching silicon. Watanabe et al., U.S. Pat. No. 5,112,437 disclose solutions of hydrogen fluoride gas in methanol for removing silicon oxide films from semiconductor substrates.
Buffered oxide etchants (xe2x80x9cBOEsxe2x80x9d) consisting of solutions of ammonium fluoride (or polyalkylammonium fluorides) and hydrofluoric acid. Ohmi et al., U.S. Pat. No. 5,277,835, disclose aqueous solutions of hydrogen fluoride and ammonium fluoride for surface cleaning of semiconductor wafers. Kinsbron et al., U.S. Pat. No. 4,343,677, utilize ammonium fluoride/hydrofluoric acid in a molar ratio of about 10:1 in water/ethylene glycol solvent for patterning silicon dioxide films. Roche, U.S. Pat. No. 4,921,572, discloses aqueous etchant solutions made from water, hydrogen fluoride, and ethylenediammonium difluoride or diethylenetriammonium trifluoride for etching of silicon dioxide.
Aqueous scions of ammonium fluoride, Scardera and Roche, U.S. Pat. No. 4,871,422, disclose solutions comprised of ammonium fluoride and wetting agents in water for etching silicon dioxide layers.
Non aqueous solutions of ammonium fluoride, Bowden and Switalski, U.S. Pat. No. 5,320,709, disclose an etchant comprised of a solution of anhydrous ammonium fluoride in ethylene glycol for removing organometallic and organosilicon residues.
Related solutions, Ward et al., U.S. Pat. No. 5,571,447 disclose stripping compositions containing fluoboric acid, water, polyhydric alcohols other than ethylene glycol, and fluorine-containing compounds.
All of these prior art solutions have the limitation that they etch BPSG layers and PSG layers more rapidly than thermal oxide layers. The ratio of etch rates of BPSG to thermal oxide is approximately 4:1 for all of the prior art solutions. In other words, BPSG layers are etched about four times more rapidly than thermal oxide layers. Another way of stating this is that the etch rate xe2x80x9cselectivityxe2x80x9d of BPSG to thermal oxide is 4:1. To the inventors"" knowledge there is no prior art or commercial etching solution that circumvents this limitation.
The formulations of the present invention etch doped silicon oxide, such as BPSG and PSG layers, at rates greater than or equal to the etch rates of undoped silicon oxides such as thermal oxide. The formulations have the general composition of a chelating agent, preferably weakly to moderately acidic (0.1-10%; preferably 0.2-2.8%); a fluoride salt which may be ammonium fluoride or an organic derivative of either ammonium fluoride or a polyammonium fluoride (1.65-7%; preferably 2.25-7%); a glycol solvent (71-98%; preferably 90-98%); and optionally, an amine.
It is an advantage of the present invention that articles of manufacture having doped silicon oxide layers can be etched at rates greater than or equal to the etch rates of undoped silicon oxide layers.
It is another advantage of the present invention that semiconductor devices having silicon oxide layers that include silicon dioxide, BPSG and/or PSG can be etched at a substantially uniform etch rate.
These and other objects and advantages of the present invention will become understood upon review of the following detailed description.